The present invention relates to methods for fabricating superconductive joints, particularly in multifilamentary superconductive wires or cables. Even more particularly, the present invention relates to a mold based method for superconductive joint fabrication which employs agitation of the wires in a solution for stripping the metal matrix surrounding the multifilamentary superconductive strands.
A growing list of materials has now been found to exhibit superconductive properties when are cooled to temperatures below a critical value. Below this temperature all electrical resistance disappears. This permits the maintenance of current flow in superconductive circuits without external energy or power sources. In particular, superconductive conductors disposed in the form of electrical solenoids and coils of various configurations are capable of substantially continuous operation with no requirements to add additional electrical energy to the circuit. Superconductive circuits employing such solenoids are particularly advantageous in nuclear magnetic resonance (NMR) medical diagnostic imaging and spectroscopy systems. Moreover, superconductive circuits have found utility in a number of applications including power distribution and in magnetically levitated vehicles.
In any application in which superconductive wire is employed, it is almost variably necessary to employ one or more superconductive joints. However, to ensure that the resulting closed loop or circuit is entirely superconductive, it is necessary to ensure that the joint between the wire ends is also superconductive. However, superconductive materials display a tendency to undergo sudden and unexpected transitions to the resistive or ohmic state from the superconductive state. This phenomenon is referred to as quenching. The reasons for this phenomenon are not thoroughly understood. However, it is strongly believed that localized heating effects contribute to the phenomenon. However, the precise physical reasons for quenching do not yet appear to be fully understood. Accordingly, methods for its prevention are best describable as empirical rather than theoretical.
While quenching phenomena can occur in any portion of a superconductive circuit, it nonetheless appears that superconductive joints are in fact particularly susceptible to quench phenomena. Quenching is undersirable for at least three reasons. Firstly, quench conditions require restoration of the current in the circuit. Secondly, quench conditions often result in the undesired heating of the cryogenic fluid, typically liquid helium. Thirdly, quenching can cause damage to unprotected circuit elements. Accordingly, because of the undesired consequences of quenching and because of the particular susceptibility of superconductive joints to quench phenomena, it is seen that it is important to fabricate superconductive joints which are as immune as possible to this phenomenon.
In the case in which superconductive wires are to carry high levels of electrical current, for example 1,000 amperes and above, it is common practive to employ multifilamentary superconductive material. Typically such superconductive material comprises a carrier or matrix metal such as copper or copper-nickel alloy or a similar matrix conductor in which filaments of niobium-titanium alloy are incorporated. In such conductors an array of filaments are imbedded within a bulk carrier matrix. The formation of superconductive joints between multifilamentary wire ends poses particularly difficult problems. For example, in one form of superconductive joint the individual filaments are soldered to a superconductive sheet individually. This is a highly labor intensive operation. While this operation produces workable superconductive joints having even greater reliability against quenching.
A number of publications and patents deal with niobium-titanium superconductors and methods and means of forming joints between such superconductors.
One such publication is an article appearing in the October 1977 issue of Welding Journal starting at page 23 and titled "Soldering of Copper-Clad Niobium-Titanium Superconductor Composite" and dealing with the use of a variety of solders and fluxes. The solder joints were not superconducting. No flux was found which permitted and/or caused the solder to wet the superconducting filaments.
A method of forming a superconductive butt joint between copper-clad niobium-titanium superconductors by overwrapping the butt joint with smaller shunt superconductors and attaching the shunt in place by solder including a lead-bismuth solder is disclosed in U.S. Pat. No. 3,453,378. Various prior art methods of forming superconducting joints are disclosed in this patent as well as problems arising from failure of such joints.
The properties of various solders including solders containing lead and bismuth potentially useful in forming superconductive joints are disclosed in the article titled "Superconductivity Measurements In Solders Commonly Used for Low Temperature Research" appearing at page 180 of Reviews of Scientific Instruments, Vol. 40, January 1969.
A superconductive connection involving use of solders is described in U.S. Pat. No. 3,346,351 assigned to the same assignee as the present application.
A variety of superconductive solders and their uses are described in U.S. Pat. No. 3,156,539 also assigned to the same assignee as the subject application.
Formation of a superconductive joint employing combination of a superconductive low melting alloy containing combinations of lead-bismuth-tin and an outer crimped sleeve are taught in U.S. Pat. No. 3,449,818.
A method of making superconductive joints is also disclosed in U.S. Pat. No. 3,422,529 and is based upon the use of a crimped sleeve or cylinder which may comprise either stainless steel or a superconductive alloy. This patent however does not describe the use of solders or the multifilamentary condition and in particular requires twisting a pair of solid superconductive wires.
Accordingly, it is seen that many researchers have sought methods for forming reliable superconductive joints. It is also seen that the methods found have been sought empirically and that satisfactory explanations for the success of certain methods have not been forthcoming.